1. Field of the Invention
The present invention relates generally to novel semiconductor light emitting devices provided in the form of a single semiconductor chip and capable of emitting light of white color, a neutral color between red and blue, such as magenta, pink and the like, and the like and particularly to structures for reducing stress exerted to the same.
2. Description of the Background Art
As a material for a high bright light emitting diode (LED) a device including a light emitting layer of AlGaAs, GaAsP and the like for red color has been put to practical use and an intensity of no less than several candela (Cd) has been implemented by an inexpensive LED. Other than LEDs for red, GaP for green and yellow-green, GaInN for blue and green, AlGaInP for Mars yellow and yellow have all been put to practical use as inexpensive LEDs.
These LEDs, however, all use a single semiconductor material for their light emitting layers and in principle they can only provide monochromatic light emission. Thus, while typical LED structures are capable of emitting light of a primary color, a color between red and green, and a color between green and blue, such as red, Mars yellow, yellow, yellow-green, green, blue-green, blue, blue-violet and violet, they are incapable of emitting light of a neutral color between red and blue and a neutral color between red, green and blue.
For illumination, decoration and some other similar applications for displaying, a source of light of a color between red and blue (e.g., magenta, pink), a color between red, blue and green (i.e., white), or the like is demanded, rather than a source of monochromatic light as described above. As has been described above, typical LEDs can only achieve a source of monochromatic light, and thus a fluorescent lamp, an incandescent lamp or the like is used as a source of light but it has suffered a short lifetime, a large size and a low luminosity.
In contrast, the present inventors have succeeded in obtaining white light, pink light, magenta light and the like from a single LED using a ZnSe-based homoepitaxial layer formed on a ZnSe substrate, as has been described in Japanese Patent Laying-Open Nos. 2000-82845 and 2000-150960.
A ZnSe substrate which is doped with iodine, aluminum, chlorine, bromine, gallium, indium or the like can provide conductance of n-type, and when it is illuminated with light of shorter than 510 nm ZnSe can provide a so-called self-activated (SA) luminescence, a broad light emission having a center of an emission wavelength in a range of 550 nm to 650 nm. This light emission appears to be yellow, Mars yellow, or the like.
The central wavelength and the half-width of emission spectrum of SA luminescence can be adjusted by the species, amount or the like of the dopant introduced. Furthermore, a light emission structure of ZnSe used as a matrix can be formed on a ZnSe substrate through homoepitaxy. In this light emission structure, ZnSe or ZnCdSe or ZnSeTe can be used for an active layer to form a high bright LED emitting light at a wavelength from 460 nm to 510 nm for blue color or blue-green color.
FIG. 1 shows a concept of the present LED 5. In the figure an epitaxial light emission structure provides blue or blue-green light emission, of which that emitted toward the substrate is absorbed by a ZnSe substrate 1 and excites optically SA light emission and thus provides yellow, Mars yellow or red light. The former light emission and the latter light emission can be combined together to obtain emission of light of white, pink, magenta or other similar neutral colors.
The present LED 5 employs a group II-VI compound semiconductor, or ZnSe, as a main material. It should be noted, however, that this ZnSe crystallographically has a significantly smaller strength than GaP, GaAs and other similar group III-V compound semiconductor that are used for popular LEDs.
As such, if in a mounting process at each step an LED chip has stress introduced therein, the ZnSe substrate crystal, epitaxial light emission structure 2 and the like are partially damaged, cracked or the like and the device is thus disadvantageously significantly impaired.
Fig. 2 shows an example of a light emission apparatus having the LED 5 described above. As shown in FIG. 2, LED 5 is mounted on a resin base 3 having a patterned interconnection (or electrode) 4, with an indium layer B attached between the LED 5 and the patterned interconnection 4. LED 5 is connected to patterned interconnection 4 by a gold wire 7 and encapsulated with transparent resin 6.
The fabrication of the above inexpensive LED 5 essentially requires transparent resin 6. However, in the step of encapsulating the chip with transparent resin 6 the resin cures and contracts and thus introduces stress which significantly contributes to degradation of the device.
Furthermore, the present LED 5, as described in Japanese Patent Laying-Open No. 2001-28459, uses indium layer 8 serving as a soldering material also as a material for an ohmic electrode.
If the melting point of indium layer 8 and the softening point of transparent resin 6 are close, then, after a completed LED lamp for example reflowed or soldered to be mounted, indium layer (electrode) 8 would be affected and the value in driving voltage of the LED lamp would vary disadvantageously.
These issues are common to ZnSe-based LEDs for white color and pink, magenta and other similar neutral colors.
The present invention has been made to overcome the above issues. An object of the present invention lies, in a light emission apparatus mounted with a ZnSe-based LED chip fixed on an electrode by means of an indium layer, in reducing degradation of a device that is attributed to stress introduced when resin encapsulating the LED chip cures and contracts, and also in preventing the indium layer from being degraded and thus changing a value of a driving voltage of the LED lamp.
In accordance with the present invention a light emission apparatus in one aspect includes: an electrode; a light emitting device mounted on the electrode with an indium layer interposed therebetween, the light emitting device having a substrate formed of an n-type ZnSe single crystal, and an epitaxial light emission structure formed of a compound crystal comprising ZnSe serving as a matrix thereof, the epitaxial light emission structure being provided on the substrate and emitting light when an electric current is introduced thereinto; and resin encapsulating the light emitting device, the resin having a glass transition temperature of lower than 80 degrees centigrade.
The resin includes epoxy resin having a glass transition temperature of higher than 30 degrees centigrade and lower than 80 degrees centigrade and being transparent or having a light diffusing agent mixed therein.
In accordance with the present invention a light emission apparatus in another aspect includes: an electrode; a light emitting device mounted on the electrode with an indium layer interposed therebetween, the light emitting device having a substrate formed of an n-type ZnSe single crystal, and an epitaxial light emission structure formed of a compound crystal comprising ZnSe serving as a matrix thereof, the epitaxial light emission structure being provided on the substrate and emitting light when an electric current is introduced thereinto; and resin encapsulating the light emitting device. The resin includes a first resin that is transparent or that has a light diffusing agent mixed therein, and that covers the light emitting device, and a second resin that is transparent or that has a light diffusing agent mixed therein, and that covers the first resin, the first resin having an elasticity corresponding to an elongation of no less than 40% at room temperature, the second resin being harder than the first resin.
The first resin includes a silicone resin and the second resin includes an epoxy resin. Furthermore the present apparatus may be a so-called surface-mounted type including a base formed of an insulator, wherein the electrode includes a patterned electrode formed on the base.
The present apparatus may include a reflector provided on the base to surround the light emitting device for reflecting fluorescence received from the substrate, wherein the resin is introduced into a region surrounded by the reflector.
In accordance with the present invention a method of fabricating a light emission apparatus, in one aspect, includes the steps of: mounting a light emitting device on an electrode with an indium layer interposed therebetween, the light emitting device having a substrate formed of an n-type ZnSe single crystal, and an epitaxial light emission structure formed of a compound crystal comprising ZnSe serving as a matrix thereof, the epitaxial light emission structure being provided on the substrate and emitting light when an electric current is introduced thereinto; and dropping epoxy resin on the light emitting device and then curing the epoxy resin to encapsulate the light emitting device, the epoxy resin having a glass transition temperature of no more than 80 degrees centigrade.
In accordance with the present invention a method of fabricating a light emission apparatus, in another aspect, includes the steps of: mounting a light emitting device on an electrode with an indium layer interposed therebetween, the light emitting device having a substrate formed of an n-type ZnSe single crystal, and an epitaxial light emission structure formed of a compound crystal comprising ZnSe serving as a matrix thereof, the epitaxial light emission structure being provided on the substrate and emitting light when an electric current is introduced thereinto; covering the light emitting device with silicone resin; and transfer-molding epoxy resin to cover the silicone resin therewith.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.